The Earth’s weather and climate regime is determined by the total solar
irradiance (TSI) and its interactions with the Earth’s atmosphere, oceans
and landmasses. Evidence from both 34 years of direct satellite monitoring and historical proxy data leaves no doubt that solar luminosity in general, and TSI in particular, are intrinsically variable
phenomena. Subtle variations of TSI resulting from
periodic changes in the Earth's orbit (Milankovich cycles:
~20, 40 and 100 Kyrs) cause climate change ranging from major ice ages to
the present inter-glacial, clearly demonstrating the
dominance of TSI in climate change on long timescales. TSI
monitoring, cosmogenic isotope analyses and correlative climate data
indicate that variations of the TSI have been a significant climate forcing
during the current inter-glacial period (the last ~ 10 Kyrs.).
Phenomenological analyses of satellite TSI
monitoring results, TSI proxies during the past 400 years and the records of
surface temperature show that TSI variation has been the dominant forcing
for climate change during the industrial era. The periodic
character of the TSI record indicates that solar forcing of climate change will likely be the dominant variable contributor to
climate change in the future.

A series of Active Cavity Radiometers
(ACRs), a new generation of sensors with the precision required for
compiling a long term TSI database for climate, were developed at the
Jet Propulsion Laboratory (JPL) of the California Institute of
Technology under the direction of ACRIM
Principal Investigator, Dr. Richard C. Willson. Their use in a series
of Active Cavity Radiometer Irradiance Monitor (ACRIM) space flight
experiments has provided a precise and traceable component of the TSI
database during more than 90 % of its 34 year history. The ACRIM
Science Team moved its operation to Columbia
University in 1995 and then back to the Jet Propulsion Laboratory
under contract in 2008. The ACRIM Instrument Team, directed by ACRIMSAT
Program Manager Sandy Kwan, operates the ACRIMSAT satellite, ACRIM
instrument and ground
telemetry instrumentation at JPL.

A
contiguous TSI database of satellite observations extends from late 1978 to
the present, covering more than three solar magnetic
cycles. It's comprised of the observations of seven independent experiments: Nimbus7/ERB1, SMM/ACRIM12, ERBS/ERBE3, UARS/ACRIM24, SOHO/VIRGO5, ACRIMSAT/ACRIM36 and SORCE/TIM7. A composite database combining these results using
overlapping, in-flight comparisons has begun to provide new insights into
the variability of total solar irradiance and its implications for climate change.

Monitoring TSI
variability is clearly an important component of climate change research,
particularly in the context of understanding the relative forcings of
natural and anthropogenic processes. The requirements for a long-term,
climate TSI database can be inferred from a National Research Council study
which concluded that gradual variations in solar luminosity of as little as
0.1 % was the likely forcing
for the ‘little ice age’ that persisted in varying degree from the late 14th
to the mid 19th centuries. A centuries-long TSI database will
have to be calibrated by either precision or accuracy to a small fraction
of this value to be of use in assessing the magnitude of solar forcing.
The current TSI database is shown in Figure 1.

Instrumentation
used in spaceflight TSI monitoring to date has utilized sensors operating
at ambient temperature (~ 20C). They provide the only practicable satellite
instrument technology presently available for extended flight experiments. The
‘native scale’, on which the results of each experiment’s TSI observations
are reported, is based on the metrology
of their individual cavity sensor properties in the International System of
units (SI).

The de-facto, redundant, overlap TSI monitoring approach that has provided a contiguous record since 1978 resulted from the deployment
of multiple, overlapping TSI satellite experiments. The traceability of this database is at the mutual precision level of overlapping experiments. This is typically
orders of magnitude smaller than the ‘absolute uncertainty’ of observations in
the international system of units (SI).
ACRIM3 results have demonstrated a residual annual traceability of ~ 5 ppm during
its 13 year mission. A carefully implemented redundant, overlap
strategy should therefore be capable of producing a climate timescale
(decades to centuries and longer) TSI record with useful traceability for assessing
climate response to TSI variation.

• A redundant,
overlapping TSI measurement strategy using existing ‘ambient temperature’
instrumentation can provide the long term traceability required by a
TSI database for climate change on climate time scales.

The state of the art measurement uncertainty for flight observations on an ‘absolute scale’ in the international system of units (SI) has not been
demonstrated to be significantly less than 1000 parts per million (ppm). The results of TSI monitoring experiments are reported on their 'native scales' as defined in SI by the ‘self-calibration’ features of their sensor technologies. Systematic uncertainties in the metrology used to relate their observations to SI caused the ± 0.25 % spread of results during the first decade of monitoring. The tighter clustering of results after 1990 is
attributable to dissemination of more accurate sensor metrology among
the various experiments and national standards labs.

A new
approach to calibrating TSI sensors has been developed by several
laboratories including
Absolute Cryogenic Infrared Radiometry at
the at the National Institute of Standards and Technology
(NIST) LBIR facility and the TSI Radiometer Facility(TRF)
at the Laboratory for Atmospheric and Space Physics (LASP) of the
University of Colorado. High powered lasers are calibrated in SI units using
self-calibrating cryogenic irradiance detectors, similar in design to the
self-calibrating ambient temperature sensors employed by satellite TSI
monitors, but operated at LHe temperature. Self-calibrating irradiance
sensors are thermal detectors that compare the heating effects of solar
irradiance and electrical heating on a cavity detector. The uncertainty of
their ability to define irradiance in SI units is temperature dependent.
When cooled to LHe temperatures self-calibrating irradiance sensors can
define irradiance at the 1 TSI level with uncertainties approaching a few
hundred ppm. The calibrated lasers are used as transfer standards to
irradiate ambient temperature satellite TSI sensors and compare their basic
'self-calibrated' SI scales to that defined by the LHe cryogenic detector's.
The effects of scattering and diffraction on sensor calibrations can also be
determined by varying the beam size of the laser. The SI uncertainty of TRF
calibrations can be on the order of 500 parts per million (ppm) or less,
with the SI scale traceable to NIST. However, the ability of satellite TSI
sensors to reproduce TRF calibrations on orbit has yet to be determined
experimentally. The LASP TSI Radiometry Facility (TRF) has been used to
calibrate TSI sensors equivalent to the SORCE/TIM, ACRIMSAT/ACRIM3 and
SOHO/VIRGO satellite sensors. The results calibrate the basic scale of these
sensors' operation in SI as well as the scattering and diffraction effects of their
field-of-view defining instrumentation.

Preliminary
LASP/TRF testing of ACRIM3 flight backup instrument found a net ~ 5000 ppm difference between ACRIM3
and the TRF cryo-radiometer defined SI scale caused by scattering (~ 3500
ppm), diffraction (~1200 ppm) and a basic SI scale difference (~300 ppm).
Application of the TRF corrections to the ACRIM3 observations has resulted
in close scale agreement with those of the SORCE/TIM experiment. Similar
results have been obtained for the SOHO/VIRGO instrument. Additional testing
is planned to decrease the uncertainties of the ACRIM3 results and apply the same
type of TRF characterizations to the results of
representative sensors of the SMM/ACRIM1 and UARS/ACRIM2 satellite
instruments.

• Preflight
calibration of satellite TSI monitors does not guarantee in-flight
observations with the laboratory level of SI uncertainty. Flight experiments
with LHe cryogenic sensors need to be implemented to calibrate the effects of
launch and flight environments on preflight calibrations.